The Pyrenean Mountain Dog or Chien de Montagne des Pyrénées is a French breed of livestock guardian dog; in France it is commonly called the Patou. It originates from the eastern or French side of the Pyrenees Mountains that separate France and Spain and is recognised as a separate breed from the Mastín del Pirineo or Pyrenean Mastiff from the Spanish side of the mountains, to which it is closely related.

The Patou is widely used throughout France as a livestock guardian, particularly in the French Alps and the Pyrenees, protecting flocks from predation by wolves and bears. It is known as the Great Pyrenees in the United States, where it is also used to protect flocks from various predators.

The Pyrenean Mountain Dog is a traditional breed of the Pyrenees. In France it is usually called the 'Patou'. It is sometimes claimed that its forebears – and those of the Pyrenean Mastiff – were white livestock guardian dogs brought to the area from Asia in Roman times, and thus that it is related to Maremmano-Abruzzese Sheepdog of Italy and the Kuvasz of Hungary. Genomic data places it within the same genetic clade as the Pharaoh Hound, Cirneco dell'Etna, and the Ibizan Hound.

In the seventeenth century, Madame de Maintenon and Louis, Dauphin of France, brought a dog of this type to the court of King Louis XIV, where they soon became in great demand, the King even naming it the Royal Dog of France. They came to be used by the French nobility to guard their châteaux, particularly in the south of the country. It is sometimes claimed that French settlers took these dogs with them to Canada and that they are among the forebears of the Newfoundland dog breed. In the 1830s, Pyrenean Mountain Dogs were used as one of the foundation breeds in the creation of the Leonberger.

After the extirpation of wolves from the Pyrenees in the nineteenth century, numbers of the dogs declined and by the beginning of the twentieth century the breed was on the verge of extinction. Local shepherds sold pups to eager tourists and some of these found their way to Britain, where several were registered with The Kennel Club at the beginning of the century; British interest in such a large breed declined during the First World War. The French aristocrat and dog authority Bernard Senac-Lagrange is credited with saving the breed from extinction at the beginning of the twentieth century, touring the mountains to collect the finest specimens available to form a breeding base. In 1923 Senac-Lagrange established a breed club, the Réunion des Amateurs de Chiens Pyrénées, and drew up the first breed standard; he also registered the breed as the Chien de Montagne des Pyrénées with the Société Centrale Canine in the same year. In 1946 the Real Sociedad Canina de España recognised the large white livestock guardian dogs on the western or Spanish side of the Pyrenees as the Mastín del Pirineo or Pyrenean Mastiff, with a slightly different breed standard. The Chien de Montagne des Pyrénées was definitively accepted by the Fédération Cynologique Internationale in 1955.

In the early 1930s the Pyrenean Mountain Dog was exported to North America, where it is known as the Great Pyrenees and became a favourite in the show ring in both Canada and the United States. In 1935, the American Kennel Club adopted a new breed standard that had a number of deviations from the French original that would not have been permitted in France. This standard promoted the exaggeration of certain physical features at the expense of functional form, and was later adopted by The Kennel Club of Great Britain. In 2011, to combat the perceived deterioration of show lines of the dog, the British Pyrenean Mountain Dog breed club released a brochure with instructions to show judges not to reward glamorous, heavy-bodied, short-muzzled examples of the breed over lean and muscular examples with weatherproof coats, capable of performing their original role in high mountainous regions.

The Pyrenean Mountain Dog is large and heavily built. According to the breed standard of the Société Centrale Canine, dogs stand from 70 to 80 cm (28 to 31 in) and bitches from 65 to 75 cm (26 to 30 in). Healthy adults typically weigh between 55 and 75 kg (120 and 165 lb). The head is not overly large in comparison to the body; the muzzle is long, broad and slightly pointed, the lips are not pendulous and the ears are small and triangular and hang flat to the head. The neck is short and strong, the chest broad and moderately deep, and the tail long, hanging low when the dog is at rest but curling over the back when the dog is roused. Unusually, the dewclaws on the hind legs are double; absence of these is considered a disqualifying fault for showing.

The coat is long, thick and double, providing protection from harsh weather; the long flat outer coat is particularly long around the neck, the tail and the backs of the legs, the under coat is fine and thick. The dogs are predominantly white in colour, with patches of black, badger, grey or various shades of tan found mostly on the head; badger is defined as a mixture of brown, black, grey and white hairs and is commonly seen in puppies but usually fades as the dog ages. Purebred examples of the breed with black patches are sometimes seen in litters; such colouration is considered a disqualifying fault for show dogs.

For millennia these dogs were used by shepherds throughout the Pyrenean region to protect their flocks from predation by wolves and bears; in this role they were usually fitted with a heavy iron wolf collar studded with long nails for protection when fighting off wolves. They were often used by shepherds in combination with the much smaller Pyrenean Sheepdog, the former guarding the flocks and the latter herding them. They were also used to smuggle contraband between France and Spain, carrying packs over the Pyrenees on routes impassable to humans to avoid detection by customs officials.

The Pyrenean Mountain Dog is today used in its original role as a livestock guardian for French shepherds in the French Pyrenees and the French Alps, as well as in the United States.

In the early 1980s farmers in the Massif Central and Lozère were experiencing problems with stray dogs attacking their flocks, so the French ITOVIC commenced an experiment with around 15 Pyrenean Mountain Dogs given to farmers. By the late 1980s the ITOVIC experiment had been completed and an association, APAP, had been formed with around 15 Pyrenean Mountain Dog breeders, with the objective of providing suitable livestock guardian dogs to potential farmers, and by 1991 around 100 dogs were working on farms.

In the early 1990s Italian wolves began to cross from Italy into France, where they have become established in approximately one third of its continental territories, particularly in the French Alps and Provence, but also throughout the Massif Central. Even before the presence of wolves was publicly reported in France, some farmers around the Mercantour National Park had reported unusual stock predation, which was, at the time, attributed by authorities to uncontrolled domestic dogs. Wolves are protected in France; in order to protect the livelihoods of farmers from wolf predation, since the late 1990s, the French government has subsidized various methods of protecting flocks from depredation, including electrified pasture fencing, secured electrified night pens, hiring of additional farm hands, and the purchase, training and upkeep of livestock guardian dogs. After the extirpation of wolves from France in the 1800s, livestock guardian dogs had been absent from the French Alps for over a century; when wolves resettled the country in the 1990s, the French Pyrenean Mountain Dog was the breed selected for use, as the ITOVIC trials had already been conducted with the breed, and within the country, the APAP was breeding Pyrenean Mountain Dogs specifically for the purpose.

In the mid-1990s the French government began importing European brown bears from Slovenia into the Pyrenees, in order to save the species from extirpation from the region by genetic inbreeding, as the local population had been reduced to an estimated six bears. With the increased numbers of bears in the region, local shepherds reported increases of stock losses to bear predation, particularly in the summer months, when shepherds move their flocks into the mountains to graze the summer alpine pastures. To assist the shepherds, government funding was provided to implement the same protection measures as those employed for wolves, and Pyrenean Mountain Dogs were given to farmers in the Pyrenees to guard flocks from predators. Studies conducted in the mid-2000s found shepherds who employed Pyrenean Mountain Dogs across the Pyrenees reported 90% fewer stock losses to predators than shepherds who did not employ the dogs. The re-employment of Pyrenean Mountain Dogs within the Pyrenees has not been without issues, with reports of hikers traversing the mountains being attacked by the livestock guardians protecting their flocks, leading to a bilingual pamphlet being produced to warn walkers and bikers against risky behaviours in order to decrease incidents.

In 2009 there were over 1000 Pyrenean Mountain Dogs being used to protect flocks against wolves in the Alps, and 500 protecting flocks in the Pyrenees. In 2019, French government funding was being provided for the upkeep of 4258 livestock guardian dogs throughout the country, 92% of which were in the French Alps and Provence, although it is estimated the total number of dogs being employed at the time was around 5000.

Beginning in the late 1970s, sheep farmers in the United States began employing livestock guardian dogs to protect their flocks from various predators, particularly coyotes and black bears, but also cougars and grizzly bears. Several factors influenced the move to integrate livestock guardian dogs into farming operations, including federal restrictions on the use of poisons to control predator numbers. A 1986 survey of over 400 farmers employing 763 livestock guardian dogs in the United States found 57% of them used Pyrenean Mountain Dogs, with Komondors, Akbashs, Anatolians and Maremmano-Abruzzese Sheepdogs being employed in fewer numbers.

A 2024 UK study found a life expectancy of 10.9 years for the breed compared to an average of 12.7 for purebreeds and 12 for crossbreeds. A 2005 Swedish study of insurance data found 58% of Pyrenees died by the age of 10, higher than the overall rate of 35% of dogs dying by the age of 10.

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